Silencing a Rarely Studied Brain Layer Slowed Mouse Brain Waves

Mice whose layer 6b neurons had been switched off slept and woke for the same total number of hours as normal mice, but the electrical rhythms running through their brains were slower and weaker, researchers at the University of Oxford report in eLife.

The authors call layer 6b by far the least studied of the cortex's layers. They read the recordings as evidence that it helps set brain state (the difference between waking, deep sleep and dreaming sleep) and that it shapes how the brain responds to orexin, a chemical that promotes wakefulness.

The team made long EEG recordings, which track brain-wave activity through electrodes on the skull, in mice in which a subset of these neurons had been silenced from birth. In those animals, theta waves were slower while the mice were awake and during REM sleep, the dreaming stage. The paper puts that rhythm at 6 to 9 Hz, or cycles per second. Total EEG power fell sharply, most of all during non-REM sleep. The total amount of waking and sleep did not change, and neither did the animals' response to being kept awake.

The group also infused orexin A straight into the brain's fluid-filled ventricles. Both the silenced mice and normal mice spent more time awake afterward, but in recordings taken from the back of the head, the slow waves that mark deep non-REM sleep were then weaker in the silenced animals.

Layer 6b neurons respond directly to orexin and connect widely across the brain, which the authors cite as their reason for studying the layer. The paper, by Elise J. Meijer, Zoltan Molnar and colleagues at Oxford, is open access.